There are five classes of gas liquefaction processes used worldwide for LNG (Liquefied natural gas) production.
These vary in complexity and efficiency—basic systems with lower levels of efficiency (high energy demand per unit of LNG produced) and more complex systems with higher efficiency. There is a trade off to be made between complexity (in terms of Capex and Opex) and efficiency.
The 5 classes can be categorized as simple gas expansion, enhanced expansion, single cycle refrigeration, dual cycle refrigeration, triple cycle refrigeration.
The efficiency of LNG plants can be measured in terms of the specific power demand per ton of LNG produced, which can be in the range of 250 kWh/t (kilowatt hour per ton) for the most efficient large scale modern plants, up to 600 to 700 kWh/t for small scale simple re-liquefaction and peak shaving plants.
The many processes available within these 5 classes also differ in some important ways, such as inherent safety risk, number of plants operating worldwide and suitability of offshore operations.
Within the second category of enhanced expander plants, various patented processes exist that aim in different ways to increase the efficiency of the single expander processes. These processes typically make use of “companders” (gas turbo-expanders directly coupled to gas compressors) in order to generate more cooling. Most processes use a dual stage, i.e., two levels of gas expansion, and hence gas cooling, to optimize the efficiency of the process. The refrigeration fluid can be either feed gas (Mustang design), Nitrogen (BHP, Kanfa Aragon, APCI and Statoil designs), or one nitrogen loop and one methane loop (CB&I Niche design).
The nitrogen based expander process has many attractions, especially in terms of ease of start-up and shutdown, leading to higher availability, and better inherent safety since the process does not contain large inventories of flammable refrigerants. However, their efficiency is lower than the more popular dual stage refrigerant cycle processes. Existing dual stage expander processes have specific power demands typically in the range from about 420 to about 500 kWh/t, whereas the aim of this new idea is to be able to reduce the specific power demand below 400 kWh/t.
Natural gas which is obtained in the form of a gas from gas and oil fields occurring in nature, is discharged from the terrestrial source to form a natural gas feed which requires processing before it can be used commercially. The natural gas feed enters a processing facility and is processed through a variety of operations in different installations to finally emerge as liquid natural gas (LNG) in a form which is suitable for use. The liquid gas is subsequently stored and transported to another suitable site for revaporisation and subsequent use. In the processing of the natural gas feed the gas emerging from the naturally occurring field must be first pretreated to remove or reduce the concentrations of impurities or contaminants, such as for example carbon dioxide and water or the like, before it is cooled to form LNG in order to reduce or eliminate the chances of blockage to equipment used in the processing occurring and to overcome other processing difficulties. One example of the impurities and/or contaminants are acid gases such as carbon dioxide and hydrogen sulphide. After the acid gas is removed in an acid gas removal installation, the feed gas stream is dried to remove all traces of water. Mercury is also removed from the natural feed gas prior to cooling. Once all of the contaminants or unwanted or undesirable materials are removed from the feed gas stream it undergoes subsequent processing, such as cooling, to produce LNG.
Typically, natural gas compositions will liquefy, at atmospheric pressure, in the temperature range −165° C. to −155° C. The critical temperature of natural gas is about −90° C. to −80° C., which means that in practice the natural gas cannot be liquefied purely by applying pressure, but must be also be cooled below the critical temperature.
Cooling of the natural gas feed may be accomplished by a number of different cooling process cycles, one of them involving the use of a nitrogen expander cycle in which, in its simplest form, a closed loop is employed in which nitrogen gas is first compressed and cooled to ambient conditions with air or water cooling and then further cooled by counter-current exchange with cold low pressure nitrogen gas. The cooled nitrogen stream is then expanded through a turbo-expander to produce a cold low pressure stream. The cold nitrogen gas is used to cool the natural gas feed and the high pressure nitrogen stream in a heat exchanging device. The work produced in the expander by the nitrogen expanding is recovered in a nitrogen booster compressor connected to the shaft of the expander. Thus, in this process cold nitrogen is not only used to liquefy the natural gas by cooling it but the cold nitrogen is also used to precool or cool nitrogen gas in the same heat exchanger. The precooled or cooled nitrogen is then subsequently further cooled by expansion to form the cold nitrogen refrigerant.
U.S. Pat. No. 6,412,302 disclose a dual expander niche LNG process. In this process for LNG production dual independent expander refrigeration cycles are used.
WO2009017414 in the name of Kanfa Aragon discloses a nitrogen dual expander process for producing LNG which is similar to the BHP process.
WO2009130466 and U.S. Pat. No. 7,386,996 both in the name of Statoil, discloses a nitrogen dual stage expander process, which is an improved version of the BHP process, but which is still based on two expanders.
U.S. Pat. No. 6,250,244 discloses that the gradient of the warming curve of the refrigerant can be altered by changing the flow rate of the refrigerant through the heat exchangers: specifically, the gradient can be increased by decreasing the refrigerant flow rate. It also discloses that if the nitrogen flow is split into two streams it is possible to make the nitrogen warming curve change from a single straight line into two intersecting straight line portions of different gradient. An example of such a process is disclosed in U.S. Pat. No. 3,677,019. This specification discloses a process in which the compressed refrigerant is split into at least two portions, and each portion is cooled by work expansion. Each work expanded portion is fed to a separate heat exchanger for cooling the gas to be liquefied. This causes the refrigerant warming curve to comprise at least two straight line portions of different gradient. This aids in the matching of the warming and cooling curves and improves the efficiency of the process. This specification was published over twenty years ago, and the process disclosed therein is inefficient by modern standards.
In U.S. Pat. No. 6,250,244 there is disclosed a process for liquefying a permanent gas stream, which also involves splitting the refrigerant stream into at least two portions in order to match the cooling curve of the gas to be liquefied with the warming curve of the refrigerant. The outlet of all the expanders in this process is at a pressure above about 1 MPa. The specification suggests that such high pressures increase the specific heat of the refrigerant, thereby improving the efficiency of the refrigerant cycle. In order to realize an efficiency improvement it is necessary for the refrigerant to be at, or near, its saturation point at the outlet of one of the expanders, because the specific heat is higher near to saturation. If the refrigerant is at the saturation point, then under these conditions there will be some liquid in the refrigerant that is fed to the heat exchangers. This leads to additional expense, because either the heat exchanger needs to be modified in order to handle a two-phase refrigerant, or the refrigerant needs to be separated into liquid and gaseous phases before being fed to the heat exchanger.
U.S. Pat. No. 6,250,244 in the name of BHP discloses a nitrogen dual expander process. In this process for producing LNG a single phase nitrogen refrigerant is used in such a way that the refrigerant stream is divided into at least two separate portions which are passed through separate turbo-expanders before being admitted to separate heat exchangers so that the warming curve of the refrigerant more closely matches the cooling curve of the product being liquefied so as to minimize thermodynamic inefficiencies and hence power requirements involved in operation of the method. U.S. Pat. No. 5,768,912 discloses a prior art nitrogen expander process with two parallel placed turbo expanders.